The invention relates to an electrical machine with an internally cooled rotor.
One power-limiting factor of electrical machines is the quality of the heat-loss dissipation. Particularly in embodiments in which the hot parts within the machine are not cooled directly by the cooling medium, it is difficult to make use of the active mass that is used. Specific zones in which the dissipation of heat losses is problematic are the end windings and the rotor. If it is possible to specifically pass heat from these two zones to the surrounding area, the power of an electrical machine can be increased, with the physical volume remaining the same.
DE 42 42 132 A1 discloses a cooling system for an electrical machine which is in the form of an internal cooling circuit system. In the case of this internal cooling, it is possible for flow to pass axially through the rotor cooling channels in both directions. A machine-internal cooling circuit is generated by using the rotor cooling channels for flow to pass through in two directions. In order to make this cooling circuit operate, two fans are required, on each of the end faces of the rotor.
DE 44 43 427 C2 discloses an internal cooling circuit system in which flow passes through the rotor from one end. The cooling air flow is blown against the end winding by means of fan blades which are provided on the rotor and located radially within the end winding. The cooling air flows via cooling channels, which are arranged on the stator circumference, to the other machine end, where it passes via the end winding to the rotor, and finally enters the rotor cooling channels, from which it is passed to the fan blades again.
The invention is based on the object of improving the cooling performance of an electrical machine with an internally cooled rotor, without using a fan impeller mounted separately on the rotor.
This object is achieved by an electrical machine having a laminated stator core in a housing and having a rotor which electrical machine has an internal cooling circuit comprising rotor cooling channels which pass axially through the rotor and are arranged on two concentric circles, in which internal cooling circuit a gaseous coolant can circulate wherein fan blades and a first means for guiding the coolant from the rotor cooling channels on one of the two concentric circles via an end winding to the rotor cooling channels on the other of the two concentric circles are provided on a first rotor end face of the machine.
The guidance of the coolant which, for example, may be air or an inert gas through the rotor cooling channels in both directions on the various concentric circles, wherein the coolant circuit is driven by fan blades which are fitted on the rotor end face and is in this case blown directly onto an end winding by the means for guiding the coolant, results in optimum cooling in particular in those areas in which the dissipation of heat losses is problematic. In this case, the fan blades pragmatically extend axially from the rotor end face, but may also each have some other expedient form. Since the solution according to the invention does not require a separate fan impeller, no additional machining of the rotor shaft for fan impeller mounting is required here, thus making it possible to also achieve a shorter axial physical length of the electrical machine, in addition to saving costs.
In one advantageous form of the refinement, the first means for guiding the coolant is in the form of an air guide cylinder, which is arranged between the two concentric circles, extends axially from the first rotor end face to in front of the fan blades and has a part which points outwards in front of the fan blades. In this case, the air guide cylinder need not have a strictly cylindrical shape in front of the part which points outwards, but can essentially correspond to half a single-shell rotation hyperboloid. The coolant flow which emerges from the rotor cooling channels on, for example, the outer of the two concentric circles is thus passed directly to the end winding in a simple manner. That part of the air guide cylinder which is arranged axially in front of the fan blades and points outwards creates a space which extends from the output of the rotor cooling channels on the relevant rotor end face to the fan blades. This considerably improves the coolant guidance in the internal cooling circuit.
In a further advantageous embodiment, a second means, for guiding the coolant, which is formed identically to the first means for guiding the coolant is arranged on a second rotor end face. This likewise considerably improves the cooling guidance in the internal cooling circuit in the area of the end winding, on the side of the second rotor end face.
In a further advantageous embodiment, the fan blades on the first rotor end face are designed such that they feed the coolant radially outwards. The force which acts radially outwards on the coolant in this case has superimposed on it the inertial force of the coolant emerging from the rotor cooling channels to form a total force which already acts on the coolant essentially in the direction of the end winding. This further improves the through-flow since less swirling occurs in this case in comparison for example, to an axial feed in which the coolant flow is guided only by the means for guiding the coolant to the end winding.
In a further advantageous embodiment, axially extending fan blades are provided on the second rotor end face and are designed such that a coolant flow which is caused by the fan blades provided on the first rotor end face is increased. This means that the fan blades on the second rotor end face feed the coolant in a direction which points between radially and axially inwards, when the fan blades on the first rotor end face are provided for feeding in a direction between radially and axially outwards.
In a further advantageous embodiment, that part of the at least one air guide cylinder which points outwards rests directly on a respective radially running end edge of the fan blades or is axially at a short distance from these end edges. This virtually completely suppresses leakage flows with respect to the coolant flow passing through the rotor cooling channels.
In a further advantageous embodiment, that part of the at least one air guide cylinder which points outwards extends radially at least to a circumferential edge, which is located radially on the outside, of the fan blades. This avoids branching flows bypassing the end winding.
In a further advantageous embodiment, when the rotor is in the form of a squirrel-cage rotor, the fan blades are formed on the corresponding short-circuiting ring. There is therefore no need to produce the fan blades separately and to fit them separately to the rotor.
In a further advantageous embodiment, the rotor cooling channels have a cross section with a circumference which is larger than that of a circle with the same cross-sectional area. In this case, by way of example, the cross section may have a corrugated circumference or may have a polygonal shape or may be in the form of a star. This provides a larger heat transfer area for cooling the rotor.
In a further advantageous embodiment, for each concentric circle on the rotor end face, a center point of a rotor cooling channel and a fan blade are arranged radially from the inside outwards in a line. This results, so to speak, in there being a dedicated fan blade for each rotor cooling channel, thus improving the feed effect of the fan blades on the coolant.
In a further advantageous embodiment, the sum of the cross-sectional areas of the rotor cooling channels on one of the two concentric circles is equal to the sum of the cross-sectional areas of the rotor cooling channels on the other of the two concentric circles. This ensures that there is a uniform flow through the rotor cooling channels on the various concentric circles, thus avoiding swirling resulting from different flow speeds.
In a further advantageous embodiment, primary cooling channels, through which a cooling medium flows, for an outer cooling circuit are provided on the housing circumference. The dissipation of heat to a primary cooling medium, which may be gaseous or liquid, considerably improves the recooling of the coolant in the internal circuit.
In a further advantageous embodiment, the primary cooling channels are in this case arranged in a helical shape on the housing circumference. This results in more uniform cooling with respect to the housing circumference, in comparison to primary cooling channels which are provided only in the corners of, for example, a rectangular housing, resulting in a simpler design than, for example, in the case of a meandering structure of the primary cooling channels.
In a further advantageous embodiment, the pitch height between successive turns of the helically arranged primary cooling channels is in this case less in the area of the end windings than in the area between the end windings. The cooling effect is therefore particularly high in those areas in which the recooling of the coolant in the internal circuit is provided by means of the primary cooling medium.
In a further advantageous embodiment, cooling ribs are provided on the housing circumference. This results in a particularly large surface area being available for exchange of heat with the environment or the primary cooling medium.
In a further advantageous embodiment, the machine is in the form of an asynchronous machine. In this case, the fan blades can be cast directly on a short-circuiting ring of, for example, an aluminum die-cast rotor.